1. Field of the Invention
The present invention relates to a process for manufacturing a non-volatile memory (NVM) structure using a so-called “soft lithography” technique. In particular, the ensuing treatment will make explicit reference, without this implying any loss of generality, to the fabrication of a completely organic or hybrid non-volatile memory.
2. Discussion of the Related Art
As is known, non-volatile memories, i.e., memories which retain stored data even in the absence of a power supply, are by now widespread in many fields of electronics. The traditional architecture of these memories envisages the use of a floating gate as a memory element, set underneath a control-gate electrode. Operation of the device exploits coupling of the two gates, separated by a dielectric material and set on top of one another on the channel between the source and the drain. The charge accumulated in the floating gate physically constitutes the datum stored in the memory element.
With the aim to improve integration, while reducing costs, and to improve the performance in terms of read and write speed, duration, and voltages applied, in the last few years alternative non-volatile memories have been proposed, characterized by innovative materials and structures. Amongst the innovative non-volatile memories, the ones that seem to be the most promising, in terms of possibility of integration, are organic memories, which use as active material polymers or small molecules, and in particular totally organic memories (i.e., ones which use only organic materials), and hybrid memories (i.e., ones which use traditional materials, for example metal materials, in combination with organic materials). Organic memories can, in fact, potentially undergo a scaling down to the individual molecule and hence enable extremely compact dimensions to be achieved.
The organic material is generally arranged between two electrodes, which are made of metal material, or alternatively which are also made of organic material, according to simple cross-point structures, possibly arranged in vertical stacks (multilayers) in order to further increase the density of the cells. Data are stored in the individual cells exploiting switching of an electrical behavior of the organic material, in particular according to two different types of operation, based upon a ferroelectric or resistive behavior of the organic material. Between the two different types, the one that seems to be more advantageous is based upon switching of a resistive state (high or low) of the organic material as a function of electrical quantities applied thereto through the respective electrodes.
It is also known that, although current manufacturing techniques on silicon substrates based upon conventional lithography, i.e., upon the use of ultraviolet (UV) radiation sources, are technologically advanced and well consolidated, above all on account of the reliability of the devices obtained, they do not enable markedly scaled dimensions, for example less than 30-40 nm, to be achieved. In addition, achievement of such small dimensions implies extremely high production costs. The resolution of the structures that can be obtained is in any case limited by the wavelength of the radiation used, as well as by the cost of the process, which, as has been said, is the higher, the smaller the dimensions of the structures that it is desired to produce. In addition, with conventional lithography, the use of organic materials is particularly critical, because they can readily undergo damage during processes employing resist masks and chemical etching.
In order to achieve increasingly demanding scaling of the memory structures, there have consequently been proposed manufacturing techniques alternative to conventional lithography. Amongst these, very promising seems to be soft lithography, which employs rather simple mechanical processes, such as molding, stamping, and pressing. For a general review on this technology, reference may be made, for example, to Y. Xia, G. M. Whitesides, Soft Lithography, Angew. Chem. Int. Ed. 1998, 37, 550-575. Soft lithography is encountering an increasingly greater interest in so far as it enables the limit of the optical diffraction to be overcome enabling structures to be obtained that are smaller and, given the same dimensions, have a lower cost; it also enables processing of organic materials.
In summary, soft-lithography techniques (amongst which the “microcontact printing”, “replica molding (REM)”, “microtransfer molding (mMT)”, “micromolding in capillary (MIMIC)”, “solvent-assisted micromolding (SAMIM)”, “embossing”, and “injection molding”) have in common the use of molds made of “soft” material, whence the name of the technology, which are used for forming the desired structures on top of a substrate. The material most widely used for producing the molds is currently polydimethylsiloxane (PDMS), on account of its properties of strength and elasticity due to the presence of siloxane groups along the chain and methylene groups. However, other materials can be used for producing the mold, such as polyurethanes, polyamides, polymethacrylates or thermosetting resins and other types of polymers with suitable requisites.
Up to now, although potentially enabling extremely small dimensions to be achieved with a lower manufacturing cost and the use of a wide range of materials (even polymeric ones), soft-lithography techniques have not found wide use in the fabrication of electronic devices, principally due to difficulties in the formation of the various superimposed layers that generally make up these devices. In particular, it is problematical the alignment of these layers, or, in a similar way, it is problematical to subsequently align the mold with previously deposited layers. In the case, for example, of memory structures, the absence of alignment between the layers can jeopardize proper operation. Accordingly, there is a need for manufacturing processes enabling full exploitation of the potentialities of this new lithographic technology.